1. Field of the Invention
This invention relates to a textile fabric constructed from electrically conductive yarns, and in particular to a fabric from yarns varying in conductivity which are arranged in the fabric to create a conductivity gradient therein.
2. Prior Art
Textile fabrics constructed from electrically conductive fibers are well known in the art. Mariker et al., U.S. Pat. No. 4,746,541, disclose electrically conductive, acrylic fibrous material which may be in the form of staple yarns, continuous filaments or a fabric. The invention of Mariker et al. may be useful for electromagnetic interference shielding and electrostatic discharge.
Electrically conductive materials made from a conductive polymer coated textile are described in Kuhn et al., U.S. Pat. No. 4,803,096. The textile material, such as a fiber, yarn or fabric, are placed in an aqueous solution of an oxidatively polymerizable compound and an oxidizing agent, resulting in a conductive polymer being formed on the surface of the textile material. The resulting polypyrole or polyaniline covered textile material has a resistivity in the range of 50 to about 10,000,000 ohms per square.
Textile fabrics having a distribution of both conductive and non-conductive fibers throughout are disclosed in Bryant, U.S. Pat. No. 4,856,299 and Yoshida et al., U.S. Pat. No. 4,929,803. In Bryant, a conductive fiber is knitted into a fabric, such as a towel, to impart improved static charge dissipation properties to the fabric. The conductive fiber is incorporated in the fabric in both the course and wale directions to dissipate an electrical charge in any direction. Yoshida et al. provide a woven fabric in which the conductive fibers are arranged in one direction only, for example in the weft direction only. In an alternate embodiment, the conductive fibers are alternated with non-conductive fibers which act to insulate individual conductive fibers from each other. The fabric is described as having anisotropic properties since current can only be conducted in one direction of the woven lattice, the direction in which the conductive fibers run.
One of the uses of electrically conductive fabrics is as a radar absorbing material (RAM) incorporated into the body of a military aircraft or other vehicle. Additionally, in the aforementioned applications, it is desirable to minimize the radar profile of the aircraft or vehicle to avoid detection and identification. It has been proposed to provide a fabric having a conductivity gradient, thereby allowing for a smooth transition around sharp edged surfaces, changes in surface angles or changes in surface composition. Material having a conductivity gradient may also be useful to give a smooth transition around radar equipment. Methods of treating a textile material, rendered electrically conductive by a coating of a conductive polymer, to produce a gradient are disclosed in Adams, Jr. et al., pending U.S. patent application Ser. No. 07/448,035, filed Dec. 8, 1989 and Gregory et al., pending U.S. pat. application Ser. No. 07/589,125. The applications relate to water jet etching and chemical reduction of the conductive polymer coating respectively, to achieve a gradient in the previously uniformly conductive textile fabric. A drawback of foregoing inventions is that subsequent to manufacturing a textile fabric from conductive polymer coated fibers, the fabric must undergo an additional processing step, namely etching or chemical reduction to create the gradient.